1. Field of the Invention
This invention relates to a focus detecting optical system, and more particularly to a focus detecting optical system of the so-called active type which is comprised of a light projecting system for applying a beam of light toward an object and a light receiving system for receiving the reflected beam of light of the applied beam of light which returns from the object.
2. Related Background Art
A focus detecting optical system of the active type has heretofore been comprised of a light projecting system for applying a beam of light emitted from a light emission source toward an object, and a light receiving system for condensing a reflected beam of light returning from the object on a light receiving element. That is, the light projecting system is comprised of a light emission source and a light projecting lens, and the light receiving system is comprised of a light receiving element and a light receiving lens. When the position of the object changes and the distance to the object changes, the position at which an image is formed on the light receiving element changes on the basis of the parallax between the light projecting system and the light receiving system. Thus, on the basis of the change in the position at which an image is formed on the light receiving element, the distance to the object can be measured (distance measurement) and a focus position on which focusing is to be effected can be detected.
FIGS. 8A and 8B of the accompanying drawings are cross-sectional views showing the optical arrangement of a focus detecting optical system of the active type according to the prior art, FIG. 8A showing a cross-sectional view in the direction of a base length, and FIG. 8B showing a cross-sectional view of a light projecting system in a direction orthogonal to the direction of the base length.
In FIGS. 8A and 8B, the light projecting system is provided with a light emission source 1 and a light projecting lens 2. In these figures, the arrow H indicates the normal direction of the light emitting surface of the light emission source 1, and the letter K indicates the optical axis of the light projecting lens 2. Thus, design is made such that a beam of light emitted from the light emission source 1 is applied toward an object 3 through the light projecting lens 2. For the convenience of the drawing sheet, the position of the object 3 is not exactly shown.
On the other hand, the light receiving system is provided with a light receiving lens 4 and a light receiving element 5. In the figures, the letter J indicates the optical axis of the light receiving lens 4, and the letter L indicates the direction of a base length (the distance between the optical axis K of the light projecting lens 2 and the optical axis J of the light receiving lens 4). Thus, design is made such that a beam of light RO reflected by the object 3 is condensed on the light receiving element 5 through the light receiving lens 4.
The center G of the outer diameter of the light projecting lens 2 is coincident with the optical axis of the light projecting lens 2.
FIG. 9 of the accompanying drawings shows the optical path of the beam of light emitted from the light emission source 1 in the prior-art light projecting system of FIGS. 8A and 8B. In FIG. 9, the letter B indicates the paraxial image plane position of the light emission source 1 by the light projecting lens 2, the letter A indicates a farther position than B, and the letter C indicates a shorter distance position than B. It is to be understood here that the light projecting lens has its spherical aberration sufficiently corrected and the light emission source 1 can be regarded as a point light source.
FIGS. 10A to 10C of the accompanying drawings show the light intensity distributions in the direction of the base length at the positions A, B and C, respectively, of FIG. 9. In these figures, the axis of ordinates represents the light intensity and the axis of abscissas represents a direction corresponding to the direction of the base length.
Generally, in a single lens, the axis linking the centers of the paraxial curvatures of two lens surfaces is called the lens optical axis, and the center axis of the outer diameter shape of the lens is called the center of the outer diameter of the lens. To accurately detect the position of the object in the light receiving system, it is necessary that the centroid of the image of the light emission source projected onto the object by the light projecting system (hereinafter referred to as the "projected light spot") exist on a straight line parallel to the center of the outer diameter of the lens (actually the axis) without pending on any change in the position of the object.
Incidentally, referring to FIGS. 10A to 10C, it is seen that the centroid of the projected light spot exists on the center G of the outer diameter of the light projecting lens 2 without depending on the positions A to C.
Therefore, in the prior-art light projecting system, the center of the outer diameter of the light projecting lens and the optical axis of the lens have been made coincident with each other and the light emission source has been positioned on the optical axis of the light projecting lens. Also, the light emission source is varied in its emitted light intensity in conformity with the angle of the light emitted from the light emitting surface thereof, by directionality. Accordingly, in order to secure the quantity of light applied toward the object to its maximum, it has been necessary to make the normal to the light emitting surface of the light emission source coincident with the center of the outer diameter (and further the lens optical axis) of the light projecting lens.
Thus, in order to accurately detect the position of the object in the light receiving system, it has been necessary to make the center of the outer diameter of the light projecting lens coincident with the lens optical axis and position the light emission source on the optical axis of the light projecting lens. Also, in order to secure the quantity of light applied toward the object to its maximum, it has been necessary to make the normal to the light emitting surface of the light emission source coincident with the center of the outer diameter (and further the lens optical axis) of the light projecting lens.
That is, in the focus detecting optical system of the active type according to the prior art, there have been limitations in arrangement as described above between the light projecting lens and the light emission source. This has led to the inconvenience that in spite of an unused space being present around the light emission source, this space cannot be effectively used.
As previously described, from the limitations in the arrangement of the light emission source and the light projecting lens, a space which is not effectively used has been present around the light emission source. To use this space effectively, the following two methods would come to mind:
(a) A method of moving the position of the light emission source in a plane parallel to the light emitting surface thereof, or
(b) A method of moving the optical axis of the light projecting lens and the position of the light emission source by predetermined distances relative to the center of the outer diameter of the light projecting lens.
In the case of method (a), as shown in FIG. 11 of the accompanying drawings, the distance by which the centroidal position of the projected light spot deviates from the center G of the outer diameter of the light projecting lens 2 in the direction of the base length depending on the positions A, B and C of the object varies, and this becomes a cause of inaccurate distance measurement.
In the case of method (b) (disclosed, for example, in Japanese Laid-Open Patent Application No. 2-50114), as shown in FIG. 12 of the accompanying drawings, the center G1 of the outer diameter of the light projecting lens 2 is moved upwardly by a predetermined distance along the direction L of the base length relative to the optical axis K of the light projecting lens 2 and the light emission source 1. Also, the center G4 of the outer diameter of the light receiving lens 4 is moved downwardly by a predetermined distance along the direction L of the base length relative to the optical axis J of the light receiving lens 4 and the light receiving element 5.
FIG. 13 of the accompanying drawings shows the optical path of the beam of light emitted from the light emission source 1 in the prior-art light projecting system of FIG. 12. In FIG. 13, the letter B indicates the position of the paraxial image plane of the light emission source 1 by the light projecting lens 2, the letter A indicates a farther position than B, and the letter C indicates a shorter distance position than B. It is to be understood here that the light projecting lens 2 has its spherical aberration sufficiently corrected and the light emission source 1 can be regarded as a point light source.
FIGS. 14A to 14C of the accompanying drawings show the light intensity distributions in the direction of the base length at the positions A, B and C, respectively, of FIG. 13. In these figures, the axis of ordinates represents the light intensity and the axis of abscissas represents a direction corresponding to the direction of the base length.
As described above, in the case of method (b), the base length can be enlarged to thereby achieve the effective use of the space. However, as shown in FIGS. 13 and 14A to 14C, the amount by which the centroidal position of the projected light spot deviates in the direction of the base length from the center G of the outer diameter of the light projecting lens 2 depending on the positions A, B and C of the object varies, and this becomes a cause of inaccurate distance measurement.
Further, the light emission source 1 is positioned on the optical axis K of the light projecting lens 2 and the normal direction H of the light emitting surface of the light emission source 1 is coincident with the optical axis K of the light projecting lens 2. Accordingly, as is apparent from FIG. 13, by the directionality of the light emission source 1, the quantity of light applied from the light emission source 1 to the light projecting lens 2 substantially decreases relative to the quantity of light emitted from the light emission source 1.